The present invention relates to a stripline filter and, in particular, to a stripline filter having at least two tuned striplines and a directional stripline coupler located in the signal path between the input and output of the filter.
A stripline filter of this type is described in U.S. Pat. No. 3,659,205 wherein the filter is disposed between the input of a UHF tuner and a mixer diode. It is impractical in this known stripline filter to provide trimming capacitors for tuning the two tuned striplines. Moreover, if this known filter arrangement is used to transfer large amounts of power or handle lower operating frequencies, without the use of additonal trimming components, the resulting dimensions become unduly large.
It is known to reduce the dimensions of low pass stripline filters by folding the conductors which have the effect of inductances, the distances between the folds being selected so that the folds do not have an electrical effect on the filter or so that the intentionally selected small distances have defined electrical effects. Such devices are disclosed in British Pat. No. 579,414 and in German Auslegeschrift No. 1,926,501 respectively.
However, the folding of striplines acting as inductances is not sufficient to solve the problem to which the present invention is directed, particularly since striplines which act only as inductances are not usually employed in stripline filters having tuned striplines.
Accordingly, it is an object of the present invention to provide a stripline filter having dimensions which are as small as possible and to make the use of trimming components unnecessary.